During the operation of fuel injectors having a solenoid drive (also referred to as coil injectors), the individual injectors have a different temporal opening behavior as a result of electrical and mechanical tolerances and consequently variations in the respective quantity of injected fuel can occur.
The relative differences in the quantity of injected fuel from injector to injector increase as the injection time periods become shorter. These relative differences in quantity were hitherto small and of no practical importance. However, the development in the direction of smaller quantities of injected fuel and shorter time periods have resulted in it no longer being possible to ignore the influence of the relative differences in quantities of injected fuel.
The differences in quantity can be analyzed and corrected inter alia on the basis of knowing the points in time at which the hydraulic opening event occurs. The hydraulic opening phase of a high pressure injection valve is usually realized in an electrical manner by means of a so-called booster phase (in which a voltage that is higher than the battery voltage is applied to the injector (boost voltage)). The hydraulic opening event occurs in this booster phase and the subsequent idling phase. It is possible using the electrical variables: coil current and coil voltage and also the ohmic or electrical coil resistance of the fuel injector, to determine magnetic variables which can be used in different analyzing methods to obtain information relating to the opening behavior and moving behavior of the injector. One of these magnetic variables is the linked magnetic flux Ψ (Psi) that is plotted against the injector current. The accuracy of the characteristic curve and consequently also of the hydraulic opening event is in this case above all determined on the basis of knowing the accuracy of the ohmic resistance.
A precise determination of the electrical coil resistance is however costly and can only be realized using additional hardware, for example for performing a 4-pole measuring procedure or four terminal sensing procedure.